BEIRUT — The battle for Syria’s second-largest airport intensified Saturday as government troops tried to reverse recent strategic gains the rebels have made in the northeast in their quest to topple President Bashar Assad.

Assad’s forces have been locked in a stalemate with rebels in Aleppo since July when the city, the largest in Syria, became a major battlefield in the two-year-old conflict the United Nations says has killed at least 70,000 people. Rebels have been trying to capture the international airport for months.

Rami Abdul-Rahman, director of the Britain-based activist group Syrian Observatory for Human Rights, said the current fighting was focused on a section of a highway linking the airport with Aleppo, the commercial hub of the nation.

The rebels have cut off the highway, which the army has been using to transport troops and supplies to a military base within the airport complex. Rebels have made other advances in the battle for the complex in recent weeks, including capturing two army bases along the road to the airport.

The rebels also control large swaths of land outside Aleppo and whole neighborhoods inside the city, which is divided between areas controlled by the regime and others held by rebels. On Saturday, the army launched an offensive on opposition strongholds outside Damascus in an effort to dislodge rebels from areas around the capital that they have been trying to storm for weeks.

On Friday, regime forces fired three missiles into a rebel-held area in eastern Aleppo, hitting several buildings and killing 29 people, according to the Observatory. The group initially reported 14 casualties in the strike that apparently involved ground-to-ground missiles. Some bodies were recovered from the rubble of damaged buildings.

Recent rebel advances in the Damascus suburbs, combined with the bombings and three straight days of mortar attacks earlier this week marked the most sustained challenge to the heart of the capital, the seat of Assad’s power.

A suicide car bombing on Thursday near the ruling Baath Party headquarters in the heart of Damascus killed 53 civilians and wounded more than 200, according to state media. Anti-regime activists put the death toll at 61, which would make it the deadliest bombing of the revolt in the capital.

The different tolls could not be reconciled.

Nobody claimed responsibility for the attack. Car bombs and suicide attacks have been a hallmark of Jabhat al-Nusra, an Islamic militant group fighting among the rebels. Nusra fighters, the most effective group on the battlefield, have led assaults on military installations and control swaths of territory in the north, including parts of Aleppo neighborhoods.